With the recent advances in biotechnology, a large number of cytokines have been isolated and purified. Also, as their mass production has become possible, they are now expected as candidate substances for new pharmaceuticals. However, many problems remain to be solved before such expectation is realized, among which is the development of a targeting drug delivery system for the desired cytokine. To strictly control cytokine behavior in vivo is assumed to be a key to enhancement of therapeutic effects and mitigation of side effects.
For example, interferon, a kind of cytokine, is a protein possessing diverse bioactivities such as antiviral activity and cell growth inhibition. For this pharmacological action, interferon is used to treat a large number of diseases, particularly chronic active hepatitis B and C and renal cancer. However, because interferon hardly migrates to the liver when administered to the live body and because it is quickly excreted from the body, it is difficult to maintain an effective concentration of interferon in the liver. For this reason, in order for the treatment of liver disease to be effective, high-dose long-term therapy is necessary, which involves the risk of side effects.
Concerning the mechanism of action of interferon in hepatitis, a mechanism of action of interferon-.alpha. in chronic active hepatitis C, for instance, is reported. Interferon reportedly acts directly on virus-infected hepatocytes to activate intrahepatocytic 2',5'-oligoadenylate synthetase (2-5AS) to produce 2',5'-oligoadenylate, which in turn activates RNase to lyse the viral-derived RNA, thus inhibiting protein synthesis and hence suppressing hepatitis C virus proliferation [Barson, S.: Tex. Res. Bio. Med., Vol. 35, 1 (1977)].
Five types of hepatitis virus, namely types A through E, have been discovered so far, among which is RNA type hepatitis C virus (HCV), whose gene was found in post-transfusion non-A, non-B hepatitis patient serum and against which there is no effective therapy. Hepatitis C is characterized by onset of acute symptoms and frequent chronic manifestation in adults. Although this chronic hepatitis progresses slowly, spontaneous healing occurs very rarely, with liver cirrhosis or hepatoma occurring in many cases. On the other hand, mass production of interferons has become possible, and these proteins have been shown to exhibit antiviral activity in vitro against HCV-related RNA type virus [Yasuyuki Ninomiya et al.: Clinical Report, 19, 231 (1985)] and to have a prophylactic effect against virus-infected mice [M. Kramer et al.: J. Interferon Res., 3, 425 (1983)], leading to expectation for their clinical effect on hepatitis C. Actually, recombinant interferon-.alpha. and -.beta. have an excellent therapeutic effect on hepatitis C patients [J. H. Hoofnagle, et al.: N. Eng. J. Med., 315, 1575 (1986)], making possible positive approach toward the treatment of chronic hepatitis, which tends to be negative; they are now widely used clinically [Masami Yamanaka et al.: Journal of the Japanese Society of Internal Medicine, 79, 1037 (1990); Sadashi Shoji et al.: Japanese Journal of Gastroenterology, 88, 706 (1991)].
However, the efficacy rate of interferon in hepatitis C remains unsatisfactory, at most 40% [S. Kakuma et al.: Am. J. Gastroenterol., 85, 655 (1990); Hepatitis Study Group, KAN TAN SUI/Journal of Liver, Gall-bladder and Pancreas, 22, 491 (1991)]. In particular, in cases of high viral contents, such as patients of the HCV II genotype, the efficacy rate is not higher than 20% [K. Yoshioka et al.: Hepatology, 16, 293 (1982)]. It may therefore be possible to improve the efficacy rate in these cases by increasing the dose or extending the administration period. Also, almost no effect is expected from the present levels of interferon dose and administration period in cases where chronic hepatitis less progressed to liver cirrhosis.
When intravenously, intramuscularly or subcutaneously administered, interferon hardly migrates to the liver, the target organ, since interferon has a short half-life in the blood. Considerable doses of interferon are therefore required. Side effects of interferon include flu-like symptoms with fever, headache and general malaise and decrease in leukocytes and platelets in the initial stage, sustained slight fever, anorexia, insomnia and tendency toward depression in the intermediate stage, and alopecia and thyroid hypofunction in the last stage. For this reason, even in those patients who should be dosed with a sufficient interferon for a considerable period of time to obtain the desired drug effect, medication should be discontinued or the dose should be lowered upon onset of such side effects.
Meantime, Japanese Patent Unexamined Publication No. 152393/1988 suggests that polyethylene glycol derivatives having a sugar chain can be used to modify cytokines, and that the resulting modified protein can be used to increase the cytokine sustainability in vivo or to improve cytokine delivery to particular cells or tissue. Also, Japanese Patent Unexamined Publication No. 211099/1992 describes a glycosyl-protein derivative useful as a carrier for selective drug delivery to the bone marrow or brain. However, none of these publications disclose any sugar-modified cytokine useful in selective cytokine delivery to the liver.
Otsubo et al. reported that a kind of sugar-modified protein migrates to the liver and is digested in intracellular lysozyme [Drug Delivery System, Vol. 6, 13-17 (1991)]. However, no disclosure is given as to hepatic orientation of sugar-modified cytokine.